When charging or discharging is stopped, a terminal voltage of a storage battery is measured, and by this an open circuit voltage can be detected. However, in an inner equivalent circuit of the storage battery, as an impedance circuit includes a capacitive component, the terminal voltage transiently changes for a while (for example, several minutes to several hours) after charging or discharging is stopped. In order to estimate a stable open circuit voltage without waiting for a convergence of the terminal voltage, in a conventional method, after charging or discharging is stopped, by using the voltage value within a predetermined time length, a plurality of coefficients of which an approximate expression containing plural exponential functions is used are obtained by the least-square method (refer to patent literature 1 described below).